The invention relates to a cylinder lock composed of a housing and a cylinder rotatably supported therein and having a key passage for the insertable flat key which has a front part with locking notches associated with cylindrical pins arranged in openings of the cylinder for retaining the pins, and the flat key has at its both lateral faces longitudinal grooves for receiving longitudinal ribs provided on the lateral faces of the key passage for lateral profile variation, wherein the longitudinal grooves at both lateral faces of the flat key lie in parallel lines extending at a distance from one another. In such a construction of the flat key the corresponding longitudinal ribs on both lateral faces of the above mentioned key passage also lie with their free ends on parallel lines extending at a distance from one another and formed by straight lines. A free space is thereby formed in the key passage between the longitudinal ribs, in which a picking pistol can be inserted, whereby the cylinder lock can be opened without the proper key.
A cylinder lock was also proposed in which the longitudinal ribs on both lateral faces of the key passage protrude so far that they extend close to the longitudinal centerline of the key passage or cross the longitudinal centerline. Thereby a free space in the key passage is avoided so that a picking pistol cannot be inserted. The longitudinal grooves provided in the flat key for receiving the longitudinal ribs of the key passage engage however so deep into the flat key that its cross section is considerably weakened or the variation is limited.